Electrophotographic image formation typically includes the following processes:
(1) forming an electrostatic latent image on an image bearing member such as photoreceptor;
(2) adhering a charged toner to the electrostatic latent image to form a toner image on the image bearing member:
(3) transferring the toner image onto a recording medium optionally via an intermediate transfer medium; and
(4) fixing the toner image to the recording medium to output an image.
Recently, electrophotographic image forming apparatuses have been rapidly changed from monochrome image forming apparatuses to full color image forming apparatuses, and the market scale of full color image forming apparatuses has been expanded.
In full color image formation, three color images such as yellow, magenta and cyan color toner images or four color images such as yellow, magenta, cyan and black color toner images are overlaid to form a full color image.
With respect to the developing method used for such image forming apparatuses, one-component developing methods, two-component developing methods, and hybrid developing methods have been conventionally used. In this regard, in order to produce full color images having a good combination of color reproducibility and clearness, the amounts of color toners constituting color toner images are preferably proportional to the potentials of the electrostatic latent images of the color images. Therefore, among the above-mentioned developing methods, two-component developing methods have been widely used from this point of view. If the amounts of color toners constituting color toner images vary even when the potentials of the electrostatic latent images of the color images do not vary, problems such that the image density and color tones of the color images vary occur.
The factors of variation of the amounts of color toners constituting color toner images on an image bearing member include factors such that the charge quantities of the toners and/or the electric resistances of the developers vary. In addition, it is known that there is a phenomenon such that a toner image is affected by history of images previously formed on the image bearing member (i.e., ghost images of previously formed images are formed).
In order to release a developer from a developer bearing member, a technique is generally used in which odd numbers of magnets are arranged inside a development sleeve (i.e., developer bearing member) in such a manner that a pair of magnets having the same polarity are arranged below the rotation axis of the development sleeve to form a developer releasing region in which the magnetic force is substantially zero so that the developer naturally falls after being used for development. However, since the toner in the developer is used for developing an electrostatic latent image and thereby a counter charge is formed in the carrier in the developer, an image force is generated between the carrier and the developer bearing member, and thereby the developer is not normally separated from the developer releasing region of the developer bearing member in which the developer releasing pole is present. Therefore, the developer including the toner at a low concentration is fed again to the development region due to rotation of the development sleeve, thereby deteriorating the developing ability of the developer, resulting in formation of a low density image. Specifically, as illustrated in FIG. 7B, a front end image portion 91s, which is formed by the developer when the development sleeve is rotated one revolution, has a normal image density, but image portions 91a1, 91a2, and 91a3, which are formed by the developer (including the toner at a low concentration) after the development sleeve is rotated one revolution, have a relatively low image density.
In attempting to solve the problem, JP-H11-65247-A proposes a technique such that a developer drawing roller including a magnet inside thereof is arranged in the vicinity of the developer releasing region of the development sleeve to separate the developer, which has been used for development, by the magnetic force thereof. However, formation of ghost images is not caused only by defective releasing of a two-component developer. In other words, even when releasing of a two-component developer is normally performed, ghost images can be formed.
In addition, as a developer is used for development, a spent toner problem in that a film of toner is formed on the surface of the carrier of the developer, and thereby the charge quantity of the toner is changed is often caused. In attempting to prevent occurrence of the spent toner problem, a carrier coated with a resin layer having good water repellency is used, or a developing device in which a new carrier is supplied together with toner while the used carrier in the developing device is discharged to prevent variation of the charge quantity of the toner is used. The spent toner problem is typically caused when a large amount of toner is used for development while a large amount of new toner is supplied, i.e., when images with a high image area proportion are formed.
In addition, a problem in that the cover layer of a coated carrier is abraded by stress caused by agitation of the developer in a developing device is often caused particularly when images with a low image area proportion are formed. If the cover layer is abraded and the core of the carrier is revealed, a problem (hereinafter sometimes referred to as a resistance decreasing problem) in that the electric resistance of the carrier decreases, resulting in variation of image density is caused.
The present inventors recognize that a need exists for a carrier which hardly causes the spent toner problem even when images with a high image area proportion are formed and which hardly causes the resistance decreasing problem even when images with a low image area proportion are formed.